John is a very complicated text. There's signs in the text of editing via more than one author. There's basically two endings. It has a clip of text that is also found in some old Luke scrolls, but isn't always found in John Scrolls.

There's a LOT going on in John. When a scholar dates a text, they're dating it's final form. There's good reasons to believe that the first author of John used older texts when composing the work. The Signs Gospel and The Passion story. There are very well respected scholars who have proposed solid theories that date The Signs gospel to at least Paul's time, if not earlier. I've seen suggestions that put it with Q at 40-50CE.

Not everyone agrees on that. I'm just saying very smart people have made good arguments.

As to the Christology argument, the basic logic is: John adds a LOT to the Christology of Paul. So, either Paul wasn't clear about Christology in his very detailed (and numerous) Christology writings or John's author didn't fully understand Paul/expanded upon Paul's ideas. It's not an unreasonable position.